If HandlerOptions.ReplaceAttr is provided it should be honored by the handler. However, documentation on implementing handler methods seems to suggest it is optional.

• Behavior defined for slog.HandlerOptions
  
• "You might also consider adding a ReplaceAttr option to your handler, like the one for the built-in handlers."

Some handlers (e.g. phsym/zeroslog) support HandlerOptions.ReplaceAttr except for the four main fields time, level, msg, and source. When that is the case it is better to use this (WarnNoReplAttrBasic) warning.